So Amstrad were too lazy to route the NDSEL0 signal to the external connector on either of the 6128 devices. So I studied the schematics and did it myself. This started because the drive in my 6128+ is dead and I fancied using the HxC on both machines with simple cable.
So first the 6128 here pin 4 of internal connector is linked to pin 25 of external which unfortunately doesn't go anywhere on the PCB.
(http://www.xora.org.uk/wp-content/uploads/2013/07/CPC6128DriveAMod.jpg)
And now the 6128+ where pin 4 of the internal connector is linked to pin 9 of the external connector.
(http://www.xora.org.uk/wp-content/uploads/2013/07/NDSEL0Link.jpg)
The problem with doing it like that is that the internal drive will still reply to SEL0. Using an ABBA switch to swap SEL0 and SEL1 is what most users do.
Bryce.
As I said 6128+ interal drive is dead, and I never use the 6128 one anyway so I just unplugged it. But its valid to do the mod anyway as long as you dont expect and internal and external to be fitted as drive 0 at the same time.